FPGA practical development skills
Published time: 2019-11-04
FPGA device selection knowledge
The selection of FPGA devices is very important. Unreasonable selection will lead to a series of subsequent design problems, and sometimes even design failures. Reasonable selection can not only avoid design problems, but also improve the cost performance of the system and extend the life of the product. Cycles and get unexpected economic results. There are seven principles for FPGA device selection: device supply channel and development tool support, device hardware resources, device interface standard, device speed grade, device temperature class, device package, and device price.
Device supply channel and development tool support
At present, the main FPGA suppliers are Xilinx, Altera, Lattic and Actel. FPGAs are developing very fast. Many types of FPGA devices are not mainstream products. In order to improve the product life cycle, it is best to The selection of mainstream devices with sufficient supply. The mainstream devices of Xllinx are Spartan-3E, Spartan-3A, Virtex-4LX, Virtex-4 SX, Virtex-4 FX, Virtex-5 LX, Virtex-5SX, Virtex-5 FX, Spartan-6 and Virtex-6. The Spartan-3E and Spartan-3A series are mainly used for logic design and simple digital signal processing. The Virtex-4 LX and Virtex-5 LX series are mainly used in high-speed logic design, Virtex-4 SX and Virtex-5 SX series. For high-speed complex digital signal processing, the Virtex-4 FX and Virtex-5 FX series are primarily used in embedded systems.
Xilinx has an integrated development environment ISE, Altera has an integrated development environment Quartus II, and two integrated development environments support the design and development of all of our devices. The integrated development environment is not only powerful, user-friendly, but also has a number of third-party partners to provide technical support to enable higher performance. Therefore, if there are no special application requirements, it is recommended to select the devices in the two companies.
Device hardware resources
Hardware resources are an important criterion for device selection. Hardware resources include logical resources, I/O resources, routing resources, DSP resources, memory resources, phase-locked loop resources, serial transceiver resources, and hard-core microprocessor resources.
The need for logical resources and I/O resources is a concern of every designer, and is generally considered. However, problems that can be caused by excessive consumption of I/O resources and routing resources are easily overlooked. In mainstream FPGA devices, the logic resources are relatively rich, which can generally meet the application requirements. However, in a more complex digital system, the consumption of excessive I/O resources may cause two problems: the FPGA is overloaded and the device is hot, which seriously affects the speed performance, working stability and lifetime of the device. The heat dissipation problem of the device; insufficient local wiring resources, the running speed of the circuit is significantly reduced, and sometimes the design cannot be adapterd, and the design fails. According to my application experience:
(1) When doing complex digital signal processing, multipliers and dividers with higher bit numbers consume a larger amount of global routing resources;
(2) When doing logic design, the bidirectional I/O port consumes a large amount of local routing resources;
(3) In the application of designing filters using memory resources, the consumption of local wiring resources is relatively large;
(4) In applications where there are many electrical interface standards and logic is complex, the consumption of local wiring resources is relatively large.
In applications where multiplication is more common and speed performance is critical, it is best to use devices with more DSP resources, such as Altera's Statix II and Statix III series, Xilinx's Virtex-4 SX. And the Virtex-5 SX series.
The memory resources in the device are mainly used for two purposes: as a high-performance filter; to achieve small-capacity high-speed data buffering. This is a relatively valuable hardware resource. In general, there are not too many memory resources in the device. The memory capacity of the device with more memory resources is also very large, and it is used less. There are not many supply channels, and the price of the device is also very high. high. Therefore, in the device selection, it is best not to unilaterally pursue the integration of the design and choose this device, you can consider the design of the low-end device + external expansion memory.
At present, the phase-locked loop is integrated in the mainstream FPGA, and the phase locking of the clock by the phase-locked loop can make the circuit obtain more stable performance. Xilinx provides a digital phase-locked loop, which has the advantage of obtaining more accurate phase control. The disadvantage is that the lower limit operating frequency is higher, generally above 24MHz. Altera provides an analog phase-locked loop, which has the advantage of The lower limit operating frequency is lower, generally above 16 MHz. The enhanced phase-locked loop operating frequency of the mainstream devices Statix II and Statix III series is only required to be above 4 MHz. The disadvantage is that the control accuracy of the clock phase is relatively poor.
In the field of communications, the transmission of high-speed data over fiber is a relatively common solution. A1tera's Statix II GX and Statix III GX series, Xilinx's Virtex-4 FX and Virtex-5 FX series all integrate high-speed serial transceivers (Note: Xilinx V5 with T products have high-speed strings Row transceivers, some models of V4, V2P also have high-speed serial transceivers), the price of such devices is generally higher. At present, the high-performance dedicated serial transceiver chips provided by companies such as National and Maxim are not expensive. Therefore, if you only need to perform fiber-optic data transmission, you don't have to choose this device; if it is fiber-optic data transmission + logic or algorithm comparison For complex applications, it is best to compare the two options and then consider whether to use the device.
Embedded development with FPGA devices with integrated hard-core microprocessors represents one direction for embedded applications. Xilinx offers the Virtex-4 FX and virtex-5 FX family of integrated PowerPCTM devices. As device prices continue to decline, in many applications, without the added cost, the device can significantly improve system performance and reduce hardware design complexity compared to traditional FPGA+MCU applications. At this point, it is ideal to use the device.
Electrical interface standard
Currently, there are many electrical interface standards for digital circuits. In complex digital systems, multiple electrical interface standards often occur. At present, the mainstream interface standards supported by mainstream FPGA devices are: 1.5 V, 1.5-V, etc., which can meet most application design requirements.
Xilinx FPGAs are ideal for all of the Xilinx FPGAs that support the SSTL-2 Class II electrical interface standard.
Device speed grade
Regarding the selection of the speed class of the device, a basic principle is to select a device with a low speed class as far as possible to meet the application requirements. This selection principle has the following benefits:
(1) Due to the transmission line effect, devices with high speed grades are more prone to signal reflection, and the design is to spend more energy on signal integrity;
(2) Devices with high speed grades are generally used less, the price is often multiplied, and the supply channels of high-speed devices are generally less. The ordering cycle of devices is generally longer, which often delays the product development cycle and reduces The listing rate of the product.
Device temperature rating
In some applications, the device's ambient temperature adaptability is highly demanded. At this time, it should be selected in industrial grade or even military or aerospace grade devices. According to the survey, each type of FPGA of Altera has industrial grade products; each type of FPGA of Xllinx has industrial grade products, and some models of FPGAs provide military grade and aerospace grade products.
At present, the mainstream devices are packaged in the form of QFP, BGA and FB-GA. The pin density of BGA and FBGA package devices is very high. Multilayer boards must be used in the design. PCB layout is quite complicated, design cost is high, and device soldering cost is high. It is relatively high, so you don't have to use it as much as possible in the design.
However, in applications where density is very high, integration is very high, and PCB board size requirements are high, BGA and FBGA packaged devices are used as much as possible. In another case, in applications where the circuit speed is very high, it is best to use BGA and FBGA packages. These two packages are advantageous for high-speed circuit design due to the small device lead lead inductance and distributed capacitance.
Device integration continues to increase, performance continues to rise, and price declines are a general trend in the development of FPGA devices. Therefore, the selection of new devices is a basic law. Taking the Virtex-5 just introduced by Xilinx as an example, the performance is 30% better than Virtex-4, while the relative price is reduced by 35%.
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